A need exists for materials whose properties can be adjusted on-demand without requiring a change in the overall environment of the material. In the domain of “smart materials,” a wide variety of materials exist that exhibit response to stimulus, including piezoelectric (which produce electrical field and charge in response to applied stress or conversely undergo strain in response to applied electric field), magnetostrictive and electrostrictive (which exhibit strain in response to magnetic or electric field), electroactive polymers (which exhibit strain/swelling in response to application of electric charge), shape memory polymers and alloys (which exhibit strain in response to thermal or magnetic stimulus) and light-activated shape memory polymers (which exhibit strain in response to light stimulus). In each of these materials, the primary effect is change in dimension and observable change in modulus is a secondary (and minor) effect.
Material properties can be dramatically changed with chemical inputs, including pH, counterion identity and concentration, and chemospecific host/guest interactions. Responses observed include phase transitions, changes in coordination or hydrogen bonding, electrostatic repulsions/attractions, swelling/deswelling and conformational changes. A wide variety of photo-crosslinking materials are currently available and are used in applications such as photoresists for microfabrication. While these materials do exhibit substantial change in mechanical properties, the reactions are not generally reversible and/or require the addition/removal of chemical reagents. Reversability is a desired property. The addition/removal of chemical agents is an undesirable requirement.
Electro- and magneto-rheological (ER and MR) fluids are known for their reversible changes in viscosity due to applied electrical or magnetic field, however the effects disappear when the stimulus is removed, and they do not affect the elastic properties of the material. Magnetorheological elastomers are a derivative of MR fluids in which the magnetic materials are bound in an elastomeric medium so that application of magnetic field changes the elastic properties of the composite structure. The stimulus is magnetic (not electrical) and it does not exhibit a power-off hold state. That is, when the magnetic field is removed, the material reverts to its base (soft) state.
Polyelectrolyte-based hydrogels (electroactive polymers, EAPs), have the ability to behave as artificial muscles which bend directionally when a potential gradient is applied. The “bending” behavior is driven by ion migration and osmotic pressure and can, therefore, occur only during the actual application of electrical energy.
Another desired property is the maintenance of a three-dimensional shape in all states. Electrically-stimulated polymeric materials that exhibit mechanical property changes other than osmotically-controlled mechanical actuation are generally stimulated either as cast films (not macroscopic in all dimensions), or they undergo a transformation between sol and gel states (shape is neither controlled nor maintained).
Forming and breaking polymer chain crosslinks can change bulk mechanical properties. However, few of these materials are reversible and of those that are, all have stimulus-defined limitations. For example, many systems are not self-contained—they require manual addition and removal of solvents or chemicals for each response. Other systems are stimulated by temperature which is difficult to direct to a specific location in the material. Moreover, the required activation temperatures could prove impractical to access and/or implement for specific applications.
The fundamental redox properties and complexation differences of iron and copper in multiple oxidation states have been reported to introduce crosslinks into linear polymers. These systems are soluble liquids in one oxidation state and dimensionally undefined gels formed by kinetic precipitation in the other. Example systems have demonstrated that either electrochemistry or light can be used in the Fe2+/Fe3+ redox couple to induce a sol-gel transition in poly(acrylic acid).
Polyelectrolytes systems can be chemically or electrochemically switched between two states by exploiting redox sensitive couples such ferrocene/ferrocenyl and Fe(CN)64−/Fe(CN)63−. The observed swelling/deswelling and aggregation/deaggregation behaviors of these systems originate from the differences in intra- and interchain electrostatic interactions caused by the change in overall charge on the metal complexes rather than by changes of coordination at the metal center.
While modulus change is inherent in the materials listed above, it is not the primary feature of many of the materials. For example, the shape memory materials have an inherent transition temperature (or magnetic field), which if exceeded, the material will rearrange its structure, thereby undergoing substantial strain (up to 8% for alloys and up to 100% for polymers). There is an accompanying change in elastic modulus (which depends on loading conditions and may be 2-3 times for alloys and orders of magnitude for polymers). The fact that the modulus change cannot be controlled independently of the dimensional change makes it impractical as a useful feature, except in very limited applications. The modulus change has been used more extensively in shape memory polymers than in alloys, partly because it is more pronounced in the polymers, but more importantly because the stresses that can be supported by the polymers are much lower than in the alloys, so if constrained the modulus change is the more dominant effect. For both of these systems, though, the more limiting factor is the thermal stimulus, which is difficult to control spatially, results in a very slow response time for material transition, and has no ability to hold the materials in the soft modulus with no power (note that for magnetic shape memory alloys, the response time is much faster, but the lack of power-off hold is still problematic as is the geometric issue of applying the stimulus).